Speciation of Manganese in a Synthetic Recycling Slag Relevant for Lithium Recycling from Lithium-Ion Batteries

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Article Speciation of Manganese in a Synthetic Recycling Slag Relevant for Lithium Recycling from Lithium-Ion Batteries

Alena Wittkowski 1, Thomas Schirmer 2, Hao Qiu 3 , Daniel Goldmann 3 and Ursula E. A. Fittschen 1,*

1 Institute of Inorganic and Analytical Chemistry, Clausthal University of Technology, Arnold-Sommerfeld Str. 4, 38678 Clausthal-Zellerfeld, Germany; [email protected] 2 Department of Mineralogy, Geochemistry, Salt Deposits, Institute of Disposal Research, Clausthal University of Technology, Adolph-Roemer-Str. 2A, 38678 Clausthal-Zellerfeld, Germany; [email protected] 3 Department of Mineral and Waste Processing, Institute of Mineral and Waste Processing, Waste Disposal and Geomechanics, Clausthal University of Technology, Walther-Nernst-Str. 9, 38678 Clausthal-Zellerfeld, Germany; [email protected] (H.Q.); [email protected] (D.G.) * Correspondence: ursula.ﬁ[email protected]; Tel.: +49-5323-722205

Abstract: Lithium aluminum oxide has previously been identiﬁed to be a suitable compound to recover lithium (Li) from Li-ion battery recycling slags. Its formation is hampered in the presence

of high concentrations of manganese (9 wt.% MnO2). In this study, mock-up slags of the system Li2O-CaO-SiO2-Al2O3-MgO-MnOx with up to 17 mol% MnO2-content were prepared. The manganese (Mn)-bearing phases were characterized with inductively coupled plasma optical emission spectrometry (ICP-OES), X-ray diffraction (XRD), electron probe microanalysis (EPMA), and X-ray

absorption near edge structure analysis (XANES). The XRD results confirm the decrease of LiAlO2 phases from Mn-poor slags (7 mol% MnO2) to Mn-rich slags (17 mol% MnO2). The Mn-rich grains are predominantly present as idiomorphic and relatively large (>50 µm) crystals. XRD, EPMA and XANES suggest that manganese is present in the form of a spinel solid solution. The absence of light Citation: Wittkowski, A.; Schirmer, T.; elements besides Li and O allowed to estimate the Li content in the Mn-rich grain, and to determine Qiu, H.; Goldmann, D.; Fittschen, U.E.A. 2+ 3+ a generic stoichiometry of the spinel solid solution, i.e., (Li(2x)Mn (1−x))1+x(Al(2−z),Mn z)O4. The Speciation of Manganese in a coefficients x and z were determined at several locations of the grain. It is shown that the aluminum Synthetic Recycling Slag Relevant for x z Lithium Recycling from Lithium-Ion concentration decreases, while the manganese concentration increases from the start ( : 0.27; : 0.54) Batteries. Metals 2021, 11, 188. to the end (x: 0.34; z: 1.55) of the crystallization. https://doi.org/10.3390/met11020188 Keywords: lithium; engineered artificial minerals (EnAM); X-ray absorption near edge structure Academic Editor: Mark E. Schlesinger (XANES); powder X-ray diffraction (PXRD); electron probe microanalysis (EPMA); melt experiments Received: 30 December 2020 Accepted: 18 January 2021 Published: 21 January 2021 1. Introduction Publisher’s Note: MDPI stays neutral Modern technologies, such as renewable energy and e-mobility, demand a new port- with regard to jurisdictional claims in folio of technology-critical elements and materials. Limited resources, national policies or published maps and institutional affil- monopolies threaten the supply of some technology-critical elements. Hence, the recovery iations. of these elements from waste is crucial. On the one hand, demand for lithium (Li) has increased rapidly due to the popularity and extraordinary performance of Li-ion batteries. On the other hand, Li is produced by mainly two countries, Australia (ca. 55%) and Chile (ca. 23%) [1]. The recycling of some components from Li-ion batteries is already put into Copyright: © 2021 by the authors. practice, e.g., cobalt, in a pyrometallurgical process [2,3]. Pyrometallurgical recycling has Licensee MDPI, Basel, Switzerland. the benefit of being adaptable to many waste streams; additionally, the emission of toxic This article is an open access article compounds like HF is prevented. Due to its ignoble character, Li is driven into the slag and distributed under the terms and is usually not recovered.The recovery of Li from pyrometallurgical recycling slags can be conditions of the Creative Commons accomplished by the targeted formation of “engineered artificial minerals” (EnAM). Attribution (CC BY) license (https:// The strategy of EnAM formation is to concentrate the elements of interest in a few creativecommons.org/licenses/by/ phases, with a structure and size that allows an efficient separation. Figure1 shows a 4.0/).

Metals 2021, 11, 188. https://doi.org/10.3390/met11020188 https://www.mdpi.com/journal/metals Metals 2021, 11, x FOR PEER REVIEW 2 of 22

can be accomplished by the targeted formation of “engineered artificial minerals” (EnAM). Metals 2021, 11, 188 2 of 20 The strategy of EnAM formation is to concentrate the elements of interest in a few phases, with a structure and size that allows an efficient separation. Figure 1 shows a schemescheme of EnAM of EnAM formation. formation. The The target target element elemen (yellowt (yellow triangle) triangle) spreads spreads over over severalseveral dif- differentferent phases phases (red (red and and green green forms) forms) and and the the matrix matrix (blue) (blue) in thein the unmodified unmodified slag slag (top (top picture).picture). The The target target element element will will be be difficult difficult to to isolate isolate due due to to itsits occurrenceoccurrence inin many dif- differentferent phases. phases. The The goal goal in in EnAM EnAM isis toto concentrateconcentrate thethe targettarget element in a single phase (red (redpentagon, pentagon, bottom picture)picture) that that differs differs physically physically and and chemically chemically from from the the other other phases phases (green(green hexagon), hexagon), and and allows allows for efficient for efficient separation separation and furtherand further treatment. treatment.

Figure 1. Sketch of engineered artificial minerals (EnAM) formation. On the left, different elements are shown in the liquid Figure 1. Sketch of engineered artificial minerals (EnAM) formation. On the left, different elements are shown in the liquid slag-matrix:slag-matrix: target target element element (yellow (yellow triangle), triang phasele), phase for EnAM for EnAM (red) (red) and otherand other phases phases (green). (green). The upperThe upper route route refers refers to an to an unmodifiedunmodified slag–the slag–the target target element element is spread is spread over over several several different different phases phases and theand matrix the matrix (blue). (blu Thee). targetThe target element element is not is not recovered.recovered. The bottomThe bottom picture picture illustrates illustrates the formation the formation of EnAM. of EnAM. The targetThe target element element is concentrated is concentrated in a singlein a single phase phase (red (red pentagon)pentagon) that differsthat differs physically physically and chemicallyand chemically from from the other the othe phasesr phases (green (green hexagon), hexagon), and allowsand allows for efficient for efficient separation separation and furtherand further treatment. treatment. Separation of artificial minerals, enriched in valuable elements from remaining slag Separation of artificial minerals, enriched in valuable elements from remaining slag components, might be carried out by flotation processes. Here, the composition and struc- components, might be carried out by flotation processes. Here, the composition and structure of phases are crucial. It can be expected that none of the artificially produced minerals ture of phases are crucial. It can be expected that none of the artificially produced minerals will show hydrophobic behavior by nature. Thus, the mineral-collector-interaction has to will show hydrophobic behavior by nature. Thus, the mineral-collector-interaction has to be studied, and this interaction relies strongly on crystal structure and ions, responsible for be studied, and this interaction relies strongly on crystal structure and ions, responsible the adsorption of the active group of collector molecules. for the adsorption of the active group of collector molecules. The recovery of Li from slags has been subject to several studies. Elwert et al. [2] found The recovery of Li from slags has been subject to several studies. Elwert et al. [2] that Li reacts with aluminum to form predominately large lithium aluminate (LiAlO2) crystals.found The that aluminum Li reacts binds with Li aluminum from the melt to form uniformly predominately in one phase large at an lithium early state aluminate of solidifying(LiAlO phase2) crystals. of the The molten aluminum slag. The binds idiomorphic Li from the to melt hypidiomorphic uniformly in lithium one phase aluminate at an early crystalsstate can of solidifying easily be separated phase of from the molten the matrix slag. by The flotation. idiomorphic Hence, to lithium hypidiomorphic aluminate islithium a aluminate crystals can easily be separated from the matrix by flotation. Hence, lithium promising EnAM candidate. The recovery of Li from LiAlO2 was successfully conducted by Haasaluminate et al. [is4 ].a Elwertpromising et al. EnAM [5] investigated candidate. The a hydrometallurgical recovery of Li from process LiAlO to2 was recover success- Li fromfully slags conducted with low by aluminumHaas et al. compared[4]. Elwert toet [al.2] [5] and investigated enriched in a silicone. hydrometallurgical However, it process to recover Li from slags with low aluminum compared to [2] and enriched in silicone. was found that the formation of LiAlO2 is suppressed in manganese (Mn)-rich slags [2]. In theseHowever, slags, it Li was is distributed found that overthe formation several phases of LiAlO of low2 is suppressed crystallinity. inElements manganese like (Mn)-rich Mn seemslags to have [2]. In a significant these slags, influence Li is distributed on the formation over several of compounds phases of low and crystallinity. grain size during Elements the formationlike Mn seem of the to slag. have Due a significant to the increasing influence amount on the of formation Li-ion batteries of compounds with Mn-based and grain cathode materials, i.e., Li-NixMnyCozO2 (NMC), it must be assumed that the Mn content in battery waste streams will increase soon. Accordingly, it is necessary to understand the role of the redox-active Mn on the genesis of crystalline, and especially amorphous grains Metals 2021, 11, 188 3 of 20

from the ionic melts. A careful characterization of the Mn-bearing grains will give insight into these processes. The first survey with mineralogical and thermodynamic methods on the formation of Li-EnAMs was conducted by Schirmer et al. [6] on a similar system, but without Mn. Their study shows that formation of spinel solid solutions is a favorable reaction with a thermodynamically proved potential to scavange Li from the slag in an early stage of the solidification. Adding Mn to this system would result in more complex solidification reactions with Mn-containing spinel solid solutions in particular, as numerous spinel-like oxides of Li and Mn are already described [7]. Therefore, in this study, Li- and Mn-bearing phases from a synthetic slag, in the following termed mock-up slag (MUS), of the system Li2O-CaO-SiO2-Al2O3-MgO-MnOx, with up to 17 mol% MnO2-content, are studied. The crystalline components are determined by powder X-ray diffraction (PXRD). The main Mn-containing phase could be identified as an oxide solid solution of spinel-type using electron probe microanalysis (EPMA). From a combination of the virtual compounds LiMnO2, Mn0.5AlO2 (1/2 galaxite spinel) and 2+ 3+ Mn0.5MnO2 (1/2 hausmannite), the formula (Li(2x)Mn (1−x))1+x(Al(2−z),Mn z)O4 was calculated. This matches with the PXRD results, as all main reflections of the oxides are located between those of LiAl5O8 and MnAl2O4. The EPMA and PXRD analysis showed that a high amount of grains are amorphous or of low crystallinity. The PXRD falls short of giving insight into the Mn species in amorphous phases. EPMA allows recognizing individual crystalline and amorphous phases. For amorphous phases, however, stoichiometric information can usually not be extracted from the data. Hence, laboratory- based X-ray absorption near edge structure (XANES) is applied here to determine the bulk Mn species of the crystalline and the amorphous components of the slag. The findings from this independent method confirmed the Mn oxidation state as being between +2 and +3, as well as the presence of Mn spinel structures.

2. Background The following section gives a short introduction to the relevant binary subsystems of the Al2O3-MnOx-Li2O ternary system, which itself is not yet published. This will allow putting the presented results into the mineralogical context. In addition, the methods used to investigate the oxidation state of Mn are brieﬂy described.

2.1. The System Li2O-MnOx with Focus on Spinel Structures

The stoichiometric spinel LiMn2O4 and several other spinel phases of Li-Mn-O are known in the Li2O-MnOx system. Paulsen and Dahn [7] created a binary phase diagram of Li-Mn-O in air. They described a spinel with the formula Li(1+x)Mn(2−x)O4 to be stable between 400 and 880 ◦C. With the rising temperature, x decreases from 1/3 at 400 ◦C to ◦ ◦ 0 at 880 C. Below 400 C, the only stable spinel phases are Li4Mn5O12 and LiMn1.75O4. Raising the temperature above 880 ◦C leads to the replacement of Li by Mn and tetragonal spinel [Li(1−x)Mnx]MnO4 [7].

2.2. The System Li2O-Al2O3

The binary system Li2O-Al2O3 exhibits several phases. Konar et al. [8] described LiAl5O8 spinel, LiAl11O17, LiAlO2 and Li5AlO4. 3+ LiAl5O8 is described as an inverse spinel structure with Al on tetrahedral sites and Li+ and the remaining Al3+ on octahedral sites. A polymorphic transition to this spinel structure occurs from the stoichiometric phase. LiAl11O17 is a high-temperature phase, ◦ only stable above 1537 C. For LiAlO2, a polymorphic transition from α- to γ-phase occurs. Li5AlO4 crystallizes below 50 mol% Al2O3, in a mixture either with LiAlO2 or with LiO2 [8].

2.3. The System Al2O3-MnOx

Solid solutions of spinel oxide type (XY2O4) with a cubic and tetragonal crystal system are described by Chatterjee et al. [9] to form in the Al2O3-MnO-Mn2O3 system. Besides Metals 2021, 11, 188 4 of 20

the cubic MnAl2O4 (galaxite), additionally, MnMn2O4 (hausmannite) with a tetragonal and cubic crystal system (transformation tetragonal -> cubic: 1172 ◦C in the air) can occur. While the tetragonal form of hausmannite incorporates only a small amount of galaxite, the cubic variety forms a solid solution.

2.4. Speciation of Oxidation States in Spinel Systems Recycling slags are usually complex elemental mixtures comprising various components and phases. Due to uncontrolled and–in geological terms–fast cooling conditions, only a few components in these slags are crystalline. Crystalline phases are easily accessible by established methods like PXRD. The amorphous components, however, elude the identiﬁcation by PXRD. Wet chemical methods like inductively coupled plasma atomic emission spectroscopy (ICP-OES) yield information on the stoichiometric composition, but fall short for the speciation. EPMA allows studying crystalline and amorphous phases. Stoichiometric information can be obtained for crystalline phases with reasonable certainty. Structure predictions on non-crystalline phases, however, are merely estimations. XANES is a convenient method to analyze the species of 3d-elements in crystalline, as well as amorphous materials. Asaoka et al. determined the oxidation state of Mn in granulated coal ash via XANES [10], while Kim et al. used XANES for Mn speciation in steel-making slags [11]. In general, the oxidation state, the coordination sphere, and in some cases the actual compound can be determined. Usually, this method is exclusively available at synchrotron sources (Sy). Here, a laboratory-based XANES spectrometer was used, which allows everyday access to measure several routine samples or improve sample preparation. The set-up is described by Seidler et al. [12,13]. Laboratory-based XANES has been used successfully in catalysis research, as well as for the determination of vanadium oxidation state in catalysts and vanadium redox ﬂow batteries [14–16].

3. Materials and Methods 3.1. Synthesis of the Mock-Up Slag The chemicals used for producing the mock-up slags are lithium carbonate (Merck KGaA purum, Darmstadt, Germany), calcium carbonate (Sigma-Aldrich, reagent grade, St. Louis, MO, USA), silicon dioxide (Sigma-Aldrich, purum p.a., St. Louis, MO, USA), aluminum oxide (Merck KGaA, Darmstadt, Germany), magnesium oxide (98%, Roth, Karlsruhe, Germany) and manganese dioxide (Merck KGaA, reagent grade, Darmstadt, Germany). The concentrations of the reactants of the slag synthesis are the following: For V-1 32 mol% Al2O3 were mixed with 16 mol% CaO, 21 mol% Li2O, 3 mol% MgO, 7.4 mol% MnO2, and 22 mol% SiO2. For V-2, 29 mol% Al2O3 were mixed with 14 mol% CaO, 19 mol% Li2O, 3 mol% MgO, 13 mol% MnO2, and 21 mol% SiO2. For V-3, 28 mol% Al2O3 were mixed with 14 mol% CaO, 18 mol% Li2O, 3 mol% MgO, 17 mol% MnO2, and 20 mol% SiO2. The chemicals were manually mixed in a mortar and ground in a disc mill for 5 min. Each sample was placed in a Pt-Rh crucible and heated in a chamber furnace (Nabertherm HT16/17, Nabertherm GmbH, Lilienthal, Germany) in an ambient air atmosphere. The temperature program is shown in Figure2. A heating rate of 2.89 ◦C/min ◦ was initially employed to reach 720 C, which is the melting temperature of Li2CO3. ◦ Subsequently, a heating rate of 1.54 C/min was used to decompose Li2CO3 and reach the target temperature of 1600 ◦C. Finally, the obtained MUS were kept at 1600 ◦C for two hours. After that, the samples were cooled to 500 ◦C at a cooling rate of 0.38 ◦C/min, and quenched in water. Three MUS V-1-3 were obtained. For PXRD and XANES measurements, parts of the obtained slag were ground for ﬁve minutes in a disc mill (Siebtechnik GmbH, Mühlheim an der Ruhr, Germany). Metals 2021, 11, x FOR PEER REVIEW 5 of 22

After that, the samples were cooled to 500 °C at a cooling rate of 0.38 °C/min, and quenched in water. Three MUS V-1-3 were obtained. For PXRD and XANES measure- Metals 2021, 11, 188 ments, parts of the obtained slag were ground for five minutes in a disc mill (Siebtechnik5 of 20 GmbH, Mühlheim an der Ruhr, Germany).

Figure 2. Temperature program of the chamber furnace. A heating rate of 2.89 ◦C/min was employed Figure 2. Temperature program of the chamber furnac◦e. A heating rate of 2.89 °C/min was ◦ to reach the melting temperature of Li2CO3 at 720 C, followed by a heating rate of 1.54 C to the employed to reach the melting temperature of Li2CO3 at 720 °C, followed by a heating rate of 1.54 target temperature of 1600 ◦C. The temperature was held for 120 min, and afterward, a cooling rate °C to the target temperature of 1600 °C. The temperature was held for 120 min, and afterward, a of 0.38 ◦C/min was employed to reach a temperature of 500 ◦C. cooling rate of 0.38 °C/min was employed to reach a temperature of 500 °C. The elemental content was determined by ICP-OES (ICP-OES 5100, Agilent, Agilent The elemental content was determined by ICP-OES (ICP-OES 5100, Agilent, Agilent Technologies Germany GmbH & Co. KG, Waldbronn, Germany). The samples were fused Technologies Germany GmbH & Co. KG, Waldbronn, Germany). The samples were fused with sodium tetraborate in a platinum crucible at 1050 ◦C, and then leached with diluted withhydrochloric sodium tetraborate acid to determine in a platinum Al, Ca, crucible Li, Mg, at Ti 1050 and °C, Si. Toand determine then leached other with elements, diluted the hydrochloricsamples were acid suspended to determine in Al, nitric Ca, acid Li, Mg, and Ti digested and Si. To at 250determine◦C and other under elements, a pressure the of samples80 bar were in an suspended autoclave (TurboWAVE, in nitric acid MLS,and digested Leutkirch at im250 Allgäu, °C and Germany).under a pressure of 80 bar in an autoclave (TurboWAVE, MLS, Leutkirch im Allgäu, Germany). 3.2. Synthesis of Galaxite via Solution Combustion Synthesis 3.2. SynthesisArtificial of Galaxite galaxite via was Solution prepared Combustion via solution Synthesis combustion synthesis. Aluminum nitrate nonahydrateArtificial galaxite (VWR chemicals,was prepared Darmstadt, via solu Germany,tion combustion analytical synthesis. reagent, Aluminum min. 98%) ni- and tratemanganese nonahydrate (II) nitrate (VWR tetrahydrate chemicals, Darmstadt, (Merck, KGaA, Germany, Darmstadt, analytical Germany, reagent, pro min. analysi, 98%) min and98.5%) manganese were used (II) nitrate as oxidizers tetrahydrate and mixed (Merck, in stoichiometricKGaA, Darmstadt, ratio Germany, 1:2. Aluminum pro analysi, nitrate minnonahydrate 98.5%) were (7.5 used g) as was oxidizers mixed withand mixed manganese in stoichiometric (II) nitrate tetrahydrateratio 1:2. Aluminum (2.5 g). ni- As a tratefuel, nonahydrate urea (Merck (7.5 KGaA, g) was Darmstadt, mixed with Germany, manganese pro (II) analysi, nitrate min.tetrahydrate 99.5%) was(2.5 g). added As a in fuel,excess urea (5(Merck g). All KGaA, components Darmstadt, were Germany, dissolved pro in water.analysi, The min. solution 99.5%) was added heated in with ex- a cessBunsen (5 g). All burner components until near were dryness. dissolved The mixturein water. was The ignitedsolution with was a heated second with Bunsen a Bunsen burner. burnerPurity until was near verified dryness. by PXRD The mixture (STOE STADI was ig P,nited STOE with & Cie a second GmbH, Bunsen Hilpertstraße burner. 10,Purity 64295 wasDarmstadt, verified by Germany). PXRD (STOE STADI P, STOE & Cie GmbH, Hilpertstraße 10, 64,295 Darm- stadt, Germany). 3.3. Powder X-ray Diffraction The analysis of the bulk mineralogical composition was provided by PXRD, using a PANalytical X-Pert Pro diffractometer, equipped with a Co-X-ray tube (Malvern Pan- alytical GmbH, Nürnberger Str. 113, 34123 Kassel, Germany). For identification of the compounds, the PDF-2 ICDD XRD database [17], the American Mineralogist Crystal Struc- ture Database [18] and the RRUFF-Structure database [19] were evaluated. Metals 2021, 11, 188 6 of 20

3.4. Electron Probe Microanalysis The analysis of single crystals and grains was performed with EPMA. EPMA is a standard method to characterize the chemical composition in terms of single spot analysis or element distribution patterns, accompanied by electron backscattered Z (ordinal number) contrast (BSE(Z)) or secondary electron (SE) micrographs. The EPMA measurements were performed on samples, which were embedded in epoxy resin, polished and coated with carbon. They were characterized using a Cameca SXFIVE FE (Field Emission) electron probe, equipped with five wavelength dispersive (WDX) spectrometers (CAMECA SAS, 29, quai des Grésillons, 92230 Gennevielliers, Cedex, France). To calibrate the wavelength dispersive X-ray fluorescence spectrometers (WDXRF), an appropriate suite of standards and analyzing crystals was used. The reference materials were provided by P&H Develop- ments (The Shire 85A Simmondley village, Glossop, Derbyshire SK13 9LS, UK and Astimex Standards Ltd., 72 Milicent St, Toronto, ON, M6H 1W4, Canada). The beam size was set to zero, leading to a beam diameter of substantially below 1 µm (100–600 nm with field emitters of Schottky-type, e.g., [20]). To evaluate the measured intensities, the X-PHI-Model was applied [21]. Li, one of the key elements in this study, cannot be directly analyzed, since EPMA uses X-ray emission to detect the elements in the sample, and the extremely low fluorescence yield and long wavelength of the Li Kα render the direct determination of this element merely impossible. With the reasonable assumption that other refractory light elements like Be and B are not present in the investigated material and volatile elements and compounds − like F, H2O, CO2 or NO3 are effectively eliminated during the melt experiment, Li can be calculated using virtual compounds. Where necessary, the balanced Li concentration was included in the matrix correction calculation. To access the analytical accuracy with respect to the determination of Li, the international reference material spodumene (Astimex Standards Ltd., Toronto, ON, Canada) and the in-house standard LiAlO2 were analyzed (Table1).

Table 1. Recovery of Li-compounds. Spod: spodumene, % StdDev: Relative standard deviation in

percent, Repeats: N = 5, R: Recovery, LiAl: LiAlO2.

Average %StDev., Ref. Average %StDev., Ref. Wt.% R (%) R (%) Spod. Spod. Spod. LiAl LiAl LiAl Al 15.04 0.35 14.4 104 41.24 0.22 40.9 101 Mg 0.00 n.a. 0.0 n.a. 0.01 n.a. 0.0 n.a. Ti 0.00 n.a. 0.0 n.a. 0.00 n.a. 0.0 n.a. Mn 0.05 n.a. 0.0 n.a. 0.00 n.a. 0.0 n.a. Fe 0.02 n.a. 0.0 n.a. 0.03 n.a. 0.0 n.a. Ca 0.01 n.a. 0.0 n.a. 0.01 n.a. 0.0 n.a. K 0.00 n.a. 0.0 n.a. 0.00 n.a. 0.0 n.a. Si 28.71 0.56 30.0 96 0.01 n.a. 0.0 n.a. Na 0.10 2.83 0.09 112 0.00 n.a. 0.0 n.a.

3.5. X-ray Absorption near Edge Structure The Mn speciation was achieved with XANES. Other than usual, XANES was not conducted at a synchrotron facility but using a laboratory-based device, the easyXES100- extended (short: easyXES100; easyXAFS LLC, Renton, WA, USA). To enable these measurements with high energy resolution comparable to that obtained at synchrotron facilities, a Rowland circle Johann-type monochromator is used, see Figure3. The instrument comprises an X-ray tube (100 W, air-cooled tube with W/Pd anode, 35 kV maximum acceler- ating potential), a spherically bent crystal analyzer (SBCA, Si 110) and an SDD detector (AXAS-M1, KETEK, Munich, Germany). The components are positioned on scissor drives. Metals 2021, 11, x FOR PEER REVIEW 7 of 22

3.5. X-Ray Absorption Near Edge Structure The Mn speciation was achieved with XANES. Other than usual, XANES was not conducted at a synchrotron facility but using a laboratory-based device, the easyXES100- extended (short: easyXES100; easyXAFS LLC, Renton, WA, USA). To enable these measurements with high energy resolution comparable to that obtained at synchrotron facilities, a Rowland circle Johann-type monochromator is used, see Figure 3. The instrument comprises an X-ray tube (100 W, air-cooled tube with W/Pd anode, 35 kV maximum ac- celerating potential), a spherically bent crystal analyzer (SBCA, Si 110) and an SDD detec- Metals 2021, 11, 188 tor (AXAS-M1, KETEK, Munich, Germany). The components are positioned on scissor7 of 20 drives.

Figure 3. Scheme of the Rowland circle monochromator. The outer circle represents the curvature Figure 3. Scheme of the Rowland circle monochromator. The outer circle represents the curvature of of the SBCA. The inner circle has a diameter matching the radius of the curvature circle. On this theinner SBCA. circle, The the inner X-ray circle source has and a diameter the detector matching are positioned. the radius The of the sample curvature is positioned circle. On right this in inner circle,front of the the X-ray detector. source The and components the detector are are set positioned. on a scissor The drive, sample allowing is positioned fine energy right inscanning. front of the detector. The components are set on a scissor drive, allowing fine energy scanning. This enables energy scanning by synchronously and symmetrically moving the de- This enables energy scanning by synchronously and symmetrically moving the detector and the source. The SBCA sits on a passively driven slider in the Rowland circle, tector and the source. The SBCA sits on a passively driven slider in the Rowland circle, which is coupled to the X-ray source stage. This way, the proper positioning of all three which is coupled to the X-ray source stage. This way, the proper positioning of all three components is elegantly achieved. A helium-filled box is installed in the beam path, to components is elegantly achieved. A helium-filled box is installed in the beam path, to lower background absorption and scattering by air. This set-up allows energy scanning lower background absorption and scattering by air. This set-up allows energy scanning with a resolution of about 1 eV. Further information about the set-up is published by Seid- with a resolution of about 1 eV. Further information about the set-up is published by ler et al. and Jahrman et al. [12,13,22]. At every energy step, an energy-dispersive X-ray Seidler et al. and Jahrman et al. [12,13,22]. At every energy step, an energy-dispersive X-ray spectrum (EDX) is acquired. The area of interest is automatically integrated by the soft- spectrum (EDX) is acquired. The area of interest is automatically integrated by the software ware based on labVIEW2017 [23]. This way, a file with the relevant information on the based on labVIEW2017 [23]. This way, a file with the relevant information on the energy energy and counts per lifetime is created and used together with the I0 measurement to and counts per lifetime is created and used together with the I0 measurement to obtain theobtain XANES. the XANES. SpectraSpectra ofof manganesemanganese dioxidedioxide (Merck(Merck AG,AG, Darmstadt,Darmstadt, Germany,Germany, forfor synthesis)synthesis) werewere recorded,recorded, validated validated with with spectra spectra from from the the literature, literature, i.e., i.e spectra., spectra from from Hokkaido Hokkaido University, Univer- Japansity, Japan [24,25 [24,25]] and subsequently and subsequently used used as reference as reference for energy for energy calibration. calibration. The scansThe scans in the in θthe-angle θ-angle space space are converted are converted to the to energy the energy space space (see Appendix(see AppendixA). A). TheThe XANESXANES ofof referencesreferences andand samplessamples werewere recordedrecorded andand processedprocessed as as follows: follows: TheThe MnMn K-edgeK-edge waswas scannedscanned fromfrom 64826482 eVeV toto 68006800 eVeV (SBCA,(SBCA, SiSi 110,110, nn == 4;4; seesee AppendixAppendixA A),), withwith aa stepstep widthwidth ofof 0.250.25 eV.eV. AtAt eacheach step,step, anan EDXEDX spectrumspectrum withwith aa 1010 ss livelive timetime waswas obtained,obtained, resulting in a a total total measurement measurement time time of of approx.approx. 3.5 3.5h per h perspectrum. spectrum. Three Three sam- samplesples of MUS of MUS V-3 were V-3 wereprepared prepared in parallel in parallel by mixing by mixing the finely the ground finely groundslag with slag Vaseline with Vaseline(ISANA; (ISANA; Dirk Rossmann Dirk Rossmann GmbH, Burgwedel, GmbH, Burgwedel, Germany) Germany) in a weight in a ratio weight of ratio2/3 Vaseline of 2/3 Vaselineand 1/3 MUS and 1/3 powder. MUS The powder. mixtures The were mixtures placed were in placedwashers in with washers a height with of a 0.1 height mm ofto 0.1adjust mm the to thickness adjust the of thickness the samples. of the The samples. washer was The sealed washer from was both sealed sides from with both adhesive sides withtape adhesive(tesapack, tape tesa (tesapack, SE, Norderstedt, tesa SE, Germany) Norderstedt, to hold Germany) the sample to hold inside, the sample and protect inside, it and protect it from environmental influences. Each replicate was analyzed three times. The following Mn species were used as references: manganese (II) oxide (oxidation state of manganese: +2; alfa aesar, Thermo fisher (Kandel) GmbH, Kandel, Germany, 99%); 2+ 3+ synthetic galaxite (Mn Al2 O4; oxidation state of manganese: +2), manganese (II, III) oxide (average oxidation state of manganese: +2.67; Sigma-Aldrich Chemie GmbH, Steinheim, 2+ 3+ Germany, 97%); braunite (Mn Mn 6[O8|SiO4]; formal oxidation state of manganese: +2.85; Friedrichroda, Thuringia, Germany; obtained from Geo collection, Clausthal University of Technology); bixbyite (Mn2O3; oxidation state of manganese: +3; Paling Mine, Republic of South Africa; obtained from Geo collection, Clausthal university of technology); and Metals 2021, 11, 188 8 of 20

manganese (IV) oxide (oxidation state of manganese: +4; Merck AG, Darmstadt, Germany, for synthesis). They were prepared in the same way as the actual sample. An additional I0 spectrum of an empty washer sealed with adhesive tape was sepa- rately acquired for every measurement to calculate the absorption coefﬁcient µ(E) according −1 to the equation: µ(E) = − ln I·I0 . The average post-edge absorption µ(E) (post edge line) in each spectrum was normalized to unity using ATHENA software [26]. Spectra from the same Mn species were merged in ATHENA and used for further analysis.

4. Results The results of the characterization of the MUS V-1-3, which are supposed to match Li-ion battery recycling slags, are presented in this section. The composition was chosen to be similar to recycling slags characterized previously by Elwert et al. [2]. The methodology applied to study the MUS chemistry comprises ICP-OES, PXRD, EPMA and XANES. Initially, the composition of the reactants and the products was determined by ICP- OES. The suppression of the formation of LiAlO2 EnAM in MUS with increasing Mn content was studied by PXRD. The microstructure and microscopic elemental composition of the MUS with the highest Mn-content (MUS V-3; MnO2: 17 mol%) were studied by BSE(Z) micrographs, as well as detailed spatially resolved quantitative point measurements and element distribution proﬁles, recorded with EPMA. From these results, a hypothesis for the genesis of the Mn-rich grains was established. This hypothesis is discussed in detail in Section 4.3. Finally, it was evaluated by studying the Mn species using XANES.

4.1. Bulk Chemistry The elemental composition of the reactants and the product slags were determined by digestion, followed by ICP-OES. The Mn-content increases from MUS V-1 to V-3, resulting in a concentration of MnO2 of 7 mol%, 13 mol%, and 17 mol% in the ﬁnal products. The results are given in Table2. A loss of about 5% of Li and 0–18% Mn occurred during the melting and cooling of the material. The MUS are therefore close in composition to the actual recycling slags studied by Elwert et al. [2].

Table 2. Comparison of the bulk chemical composition of the three melt experiments, given in mole percent.

Raw Mix (Mole Fraction) Product (Mole Fraction) Recovery % V-1 V-2 V-3 V-1 V-2 V-3 V-1 V-2 V-3

Al2O3 32 29 28 33 31 29 103 107 104 CaO 16 14 14 15 15 14 94 107 100 Li2O 21 19 18 20 18 17 95 95 94 MgO 3 3 3 3 3 3 100 100 100 MnO2 7.4 13 17 6.1 12 17 82 92 100 SiO2 22 21 20 23 22 21 105 105 105

4.2. Powder X-ray Diffraction The crystalline composition of the MUS V-1-3 was studied by PXRD and compared to an Mn-free material with comparable LiAlO2-content. The compounds gehlenite, spinel and LiAlO2 were present in all three slags (Figure4a). The Li-Al-Oxide reflections are best explained with the diffraction pattern of LiAlO2 (ICDD PDF2 no. 00-038-1464 [17]). A comparison of the reflection height of three mixtures with increasing Mn-concentration shows a negative correlation with the intensity of the LiAlO2 main reflection. A comparison with an Mn-free material with comparable Li2O content (Figure4c) indicates a strong negative influence of the Mn-concentration on the formation of LiAlO2. The low intensities of the reflection and the comparable high background imply a high amount of amorphous material being present. Metals 2021, 11, x FOR PEER REVIEW 9 of 22

4.2. Powder X-Ray Diffraction The crystalline composition of the MUS V-1-3 was studied by PXRD and compared to an Mn-free material with comparable LiAlO2-content. The compounds gehlenite, spinel and LiAlO2 were present in all three slags (Figure 4a). The Li-Al-Oxide reflections are best explained with the diffraction pattern of LiAlO2 (ICDD PDF2 no. 00-038-1464 [17]). A comparison of the reflection height of three mixtures with increasing Mn-concentration shows a negative correlation with the intensity of the LiAlO2 main reflection. A comparison with an Mn-free material with comparable Li2O content (Figure 4c) indicates a strong negative influence of the Mn-concentration on the formation of LiAlO2. The low intensities of the Metals 2021, 11, 188 9 of 20 reflection and the comparable high background imply a high amount of amorphous material being present.

Figure 4. (a) Powder X-ray diffraction (PXRD) of the solidified melt. G: Gehlenite, S: Spinel, L: LiAlO2. (b): Enlarged section Figure 4. (a) Powder X-ray diffraction (PXRD) of the solidified melt. G: Gehlenite, S: Spinel, L: LiAlO2.(b): Enlarged section of the main spinel peak. * 1: the position of the main peak of MnAl2O4 from the ICDD-PDF2 no. 00-029-0880 [17], * 2: the of the main spinel peak. * 1: the position of the main peak of MnAl O from the ICDD-PDF2 no. 00-029-0880 [17], * 2: position of the main peak of the Li(1−x)Mn2O4 spinel from the ICDD-PDF22 no.4 00-038-07891 [17], * 3: the position of the main peakthe position of the Li of2Mn the2O main4 spinel peak from of the the Li ICDD-PDF2(1−x)Mn2O4 no.spinel 01-084-1524 from the ICDD-PDF2[17], * 4: the no.position 00-038-07891 of the main [17 ],peak * 3: theof the position LiAl5O of8 spinelthe main from peak the of ICDD-PDF2 the Li2Mn2 Ono.4 spinel 00-038-1425 from the [17], ICDD-PDF2 (C): Enlarged no.01-084-1524 sections of [the17], first * 4: thetwo position main LiAlO of the2 peaks. main peak In (c of): thefor comparison,LiAl5O8 spinel the from LiAlO the2 main ICDD-PDF2 reflection no. of 00-038-1425 an Mn-free[ solidified17], (c): Enlarged melt with sections comparable of the firstLi2O twocontent main is LiAlO presented.2 peaks. In (c): for comparison, the LiAlO2 main reflection of an Mn-free solidified melt with comparable Li2O content is presented. The enlarged section of the two-theta region of the main spinel reflections gives an indicationThe enlarged of the changing section ofcomposition the two-theta of the region spinel of thewith main the spinelchange reflections of the Mn-content gives an (Figureindication 4b). of The the main changing spinel composition reflections (311) of the of all spinel three with MUS the lie change between of those the Mn-content of galaxite (Figure4b). The main spinel reflections (311) of all three MUS lie between those of galaxite spinel MnAl2O4 and LiAl5O8. In this range, two reflections of Li/Mn-spinel are located (Li1−xMn2O4 and Li2Mn2O4)[17]. This indicates the presence of a complex solid solution of 2+ 3+ a spinel-like oxide with the general formula (Li(2x)Mn (1−x))1+x(Al(2−z),Mn z)O4, which was derived by EPMA, as discussed below. In conclusion, the PXRD results show the decrease of crystalline LiAlO2 content with an increasing Mn-concentration. They also indicate the presence of a Li/Al/Mn spinel- like solid solution. The findings are consistent with the results of the microscopic EPMA analysis, which are presented in the following section. Metals 2021, 11, 188 10 of 20

4.3. Electron Probe Microanalysis

The MUS with the highest MnO2-content (MUS V-3, 17 mol%) was subjected to EPMA. From the PXRD results, it was expected that the microstructure of this sample would be most conclusive on the processes, resulting in a LiAlO2-depleted material. The main compounds of the melt experiment V-3, determined with EPMA, were: a spinel phase 2+ 3+ (Li(2x)Mn (1−x))1+x(Al(2−z),Mn z)O4; lithium aluminate (LiAl) with the stoichiometric formula Li1−x(Al(1−x)Six)O2; a gehlenite-like calcium-alumosilicate (GCAS) with the stoichiometric formula Ca2Al2SiO7 and with minute amounts of Mg and Na (max. ~0.2 wt.%); and amorphous phases (APh) with various amounts of Al, Si, Ca, Mn, small amounts of Na (<0.3 wt.%), and sometimes with unusual elements like Ba and K (contaminants enriched in the eutectic residual melt). The LiAl and the GCAS have already been described by Schirmer et al. [6] in a MUS not containing Mn but Li, Ca, Si, Al and Mg. There is strong evidence that the presence of Mn in the slag has an inﬂuence on the formation of Li and Al compounds. In particular, the formation of spinel solid solutions suggested by the PXRD is expected to inﬂuence the formation of LiAlO2. Accordingly, the EPMA focused on elucidating the genesis of the Mn- rich grains. Overall, it was found that besides negligible amounts in amorphous phases, the Mn is almost exclusively incorporated into pure oxide (spinel) structures. For this reason, a detailed examination of a representative grain of Mn-containing oxide is presented in the following. Early crystallites of this type are predominantly found throughout the whole sample. The BSE(Z) micrograph (Figure5) shows a large grain of predominantly idiomorphic Mn-enriched oxide (spinel) surrounded by idiomorphic/hypidiomorphic grains of LiAl in Metals 2021, 11, x FOR PEER REVIEW 11 of 22 a matrix of GCAS and accompanied by a grain of an APh enriched in unusual elements (contaminants), e.g., Ti and K probably representing the last eutectic melt composition.

Figure 5. Electron micrograph (BSE(Z)) of the solidiﬁedsolidified melt. Light grey grain: spinel; dark gray sections, and dendrites: LiAl, surrounded byby Ca-alumosilicateCa-alumosilicate (GCAS, (GCAS, light light grey grey sections); sections); amorphous amorphous phases phases (APh): (APh amorphous): amorphous grain grain with with unusual unu- elementssual elements (K, Ba, (K, Ti): Ba, contamination; Ti): contamination; black: black: pores pores or preparation or preparation damage. damage. The chemical The chemical analysis analysis of the of points the points marked marked in red (P294–P383)in red (P294–P383) is presented is presented in Table in3 .Table 3.

Table 3. Elemental concentrations (wt.%) at the locations depicted in Figure 5, sorted in ascending order according to the Al concentration. For a close-up of the lamellae region, see Appendix C. The distance from the lamellae region points to the nearest rim is given. Regarding the point scans, a virtual line is drawn through all points to the left rim. The distances are given from each point to the intersection of this line with the rim.

Location Lamellae Region Point Scan No. 379 382 383 380 381 374 378 377 294 Distance from 2.6 2.2 6.9 5.6 10.4 10.1 46.5 81.8 101.2 rim in µm Al 0.7 0.7 1.5 1.5 3.1 6.0 11.9 16.7 22.3 Mg 0.1 0.1 0.1 0.1 0.1 0.1 0.2 0.2 0.5 Ti 0.4 0.4 0.3 0.2 0.1 0.0 0.0 0.0 0.0 Mn 69.7 64.4 62.9 64.0 62.2 60.0 52.3 46.9 39.0 Fe 0.3 0.4 0.5 0.5 0.5 0.4 0.3 0.3 0.3 Ca 0.2 0.1 0.1 0.2 0.1 0.2 0.0 0.0 0.0 Si 0.2 0.2 0.2 0.2 0.1 0.1 0.1 0.0 0.0

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Table 3. Elemental concentrations (wt.%) at the locations depicted in Figure5, sorted in ascending order according to the Al concentration. For a close-up of the lamellae region, see AppendixC. The distance from the lamellae region points to the nearest rim is given. Regarding the point scans, a virtual line is drawn through all points to the left rim. The distances are given from each point to the intersection of this line with the rim.

The gradient of the grey shade of the Mn-oxide grain indicates an increase of the mean atomic number from the center to the rim. At the outer edge, segregation lamellae can be observed. The brighter grey shade of these lamellae indicates a higher mean atomic number than in the surrounding grain. To investigate the changing composition from the inner part of the grain to the rim, several points were analyzed. Due to the relatively small features (<1–2 µm) of the segregation lamellae, the emphasis was on the precise determination of the whiskers. Therefore, the recording of a line scan was omitted. Table3 contains the original data. The concentrations found for Al and Mn were used to calculate normalized con- 2+ 3+ tents of Al and Mn , Mn as well as the fractions of LiMnO2, Mn0.5AlO2 (1/2 galaxite spinel), and Mn0.5MnO2 (1/2 hausmannite) ﬁtting the three spinel components to the elemental amounts (Table4). The Li-concentration was subsequently obtained from the calculated amount of LiMnO2. The general formula obtained from the ﬁttings is 2+ 3+ (Li(2x)Mn (1−x))1+x(Al(2−z),Mn z)O4. The minute concentrations of the other elements are omitted in this calculation. The results show that the Al content in the grain decreases during crystallization. The result of this calculation gives an indication that a solid solution of Li-Al-Mn spinel is formed.

Table 4. Spinel formulas, calculated with the Al and Mn concentrations of Table3, sorted in ascending order according to the Al concentration. The Mn2+/Mn3+-ratio was calculated using the total measured Mn concentration in Table3. The Li

concentration is derived from the calculated fraction of LiMnO2. In the first section of the table, the concentrations of the elements are given in weight percent. In the second section, the calculated fractions of the different virtual components in 2+ 3+ percent are presented. In the last section, the stoichiometric ratio of the formula (Li(2x)Mn (1−x))1+x(Al(2−z),Mn z)O4 is presented. The distance from the lamellae region points to the nearest rim is given. Regarding the point scans, a virtual line is drawn through all points to the left rim. The distances are given from each point to the intersection of this line with the rim.

Location Lamellae Region Point Scan No. 379 382 383 380 381 374 378 377 294 Distance from rim in µm 2.6 2.2 6.9 5.6 10.4 10.1 46.5 81.8 101.2 Concentrations (wt.%) Al 0.7 0.7 1.5 1.5 3.1 6.0 11.9 16.7 22.3 Mn2+ 21.5 12.2 11.5 13.6 14.5 17.9 19.3 21.8 22.7 Mn3+ 48.2 52.2 51.4 50.4 47.7 42.1 33.0 25.1 16.7 Li 0.8 3.7 4.0 3.3 3.2 2.4 2.4 2.0 2.1 Calculated fractions (%) Mn0.5AlO2 2.1 2.2 4.7 4.9 10.0 19.2 38.2 53.5 71.5 LiMnO2 11.0 49.9 53.6 45.1 42.7 31.8 31.9 26.7 28.5 Mn0.5MnO2 86.9 47.9 41.7 50.0 47.3 48.9 29.9 19.9 0.0 2+ 3+ Stoichiometric factors (Li2xMn (1−x))(1+x)(Al(2−z),Mn z)O4 x 0.13 0.54 0.58 0.49 0.46 0.34 0.33 0.26 0.27 z 1.95 1.95 1.89 1.88 1.76 1.55 1.15 0.85 0.54 Metals 2021, 11, 188 12 of 20

4.4. X-ray Absorption near Edge Structure To verify the structures suggested from EPMA analysis, XANES was conducted on the sample MUS V-3. For comparison, the spectra of known Mn oxidation states were recorded Metals 2021, 11, x FOR PEER REVIEW 13 of 22 as well. In Figure6, Mn K-edge XANES spectra of compounds representing oxidation states from Mn2+ to Mn4+ are displayed for comparison with the spectrum of the MUS.

Figure 6. Spectra of Mn samples of different oxidation states. The spectra are compared to the spectrum of the mock-up Figureslag 6. (MUS). Spectra A shiftof Mn of samples the edge of accompanies different oxidation the increase states. in Th thee oxidationspectra are state compared which is to indicated the spectrum by the of arrow. the mock-up The mean slagoxidation (MUS). A states shift of of Mn thein edge the compoundsaccompanies are the as increase follows: in MnO: the oxidation +2; galaxite: state +2, which hausmannite: is indicated +2.67; by braunite:the arrow. +2.85; The mean bixbyite: oxidation+3; and states MnO 2of: +4.Mn in the compounds are as follows: MnO: +2; galaxite: +2, hausmannite: +2.67; braunite: +2.85; bixbyite: +3; and MnO2: +4. A shift of the edge to higher energies with increasing oxidation state is observed (Figure6); this is a well-known effect [ 27]. From the edge shift and the shape of the curve, A shift of the edge to higher energies with increasing oxidation state is observed (Fig- it can be concluded that the oxidation state of Mn in the MUS is a mixture of +2 and +3. A uremixture 6); this ofis a +4 well-known and +2 is unlikely. effect [27]. A combinationFrom the edge of shift both and would the resultshape inof athe relatively curve, it ﬂat cancurve, be concluded which is notthat observed the oxidation in the state MUS of spectrum. Mn in the For MUS a better is a mixture overview of of +2 the and correlation +3. A mixturewith oxidationof +4 and states +2 is rangingunlikely. from A combination +2 to +3, see of Appendix both wouldB. result in a relatively flat curve, whichAccording is not observed to the results in the of MUS the spectrum. Mn K-edge For XANES a better ofoverview the reference of the correlation samples, the withaverage oxidation oxidation states ranging state of from Mn has +2 to +3, be betweensee Appendix +2 and B. +3. Additionally, the analysis viaAccording EPMA and to PXRDthe results strongly of the supports Mn K-edge the presenceXANES of of the spinel reference structures samples, involving the av- Mn erageund oxidation Al. The XANESstate of spectraMn has showto be thatbetween Mn is+2 not and present +3. Additionally, in the pure formthe analysis of any ofvia the EPMAanalyzed and PXRD oxides. strongly The same supports is concluded the presence from theof spinel EPMA, structures which in involving addition hasMn shownund Al.that The theXANES Al-percentage spectra show in these that Mn Mn phases is not ispr loweresent in than the in pure pure form galaxite. of any of the analyzed oxides. The same is concluded from the EPMA, which in addition has shown that the Al-percentage in these Mn phases is lower than in pure galaxite. Accordingly, linear combinations of the spinels galaxite and hausmannite, as well as of galaxite and bixbyite, were calculated. From these linear combinations, the model XANES spectra in Figures 7 and 8 were obtained. These mixtures present a lower Al-content than galaxite, but still have a spinel structure. The results obtained from these linear combinations can be seen in Figures 7 and 8. A linear combination of hausmannite with bixbyite was also calculated. These results are shown in Appendix D.

Metals 2021, 11, 188 13 of 20

Accordingly, linear combinations of the spinels galaxite and hausmannite, as well as of galaxite and bixbyite, were calculated. From these linear combinations, the model XANES spectra in Figures7 and8 were obtained. These mixtures present a lower Al- content than galaxite, but still have a spinel structure. The results obtained from these Metals 2021, 11, x FOR PEER REVIEW 14 of 22 linear combinations can be seen in Figures7 and8. A linear combination of hausmannite with bixbyite was also calculated. These results are shown in AppendixD.

Figure 7. Linear combination of hausmannite (H) and galaxite (G). At the edge jump, the MUS spectrum fits the spectrum of hausmannite, whereas after the jump, the spectrum is more similar to galaxite. Figure 7. Linear combination of hausmannite (H) and galaxite (G). At the edge jump, the MUS spectrum fits the spectrum of hausmannite, whereas after the jump,The the linear spectrum combinations is more similar (Figures to galaxite.7 and8 ) of a 50:50 (mass) mixture of both galaxite and bixbyite, as well as galaxite and hausmannite, are quite similar to the measured spectrum of the MUS V-3. A combination of bixbyite and hausmannite is excluded from further investigation, as the shape is significantly different (see AppendixD). Accordingly, the experimental XANES data was obtained from a 50:50 (mass) mixture of galaxite and bixbyite, as well as galaxite and hausmannite. The obtained spectra are shown in Figures9 and 10. Figure 10 shows a close-up of the edge-jump. The spectra displayed in Figures9 and 10 show that the pre-edge, the edge jump and the region after the edge of the MUS and both references are similar. The results suggest that the slag contains a mixture of Mn2+ and Mn3+, confirming the EPMA analysis. The combinations of galaxite and bixbyite with an average Mn oxidation state of 2.69—as well as of galaxite and hausmannite, with an average oxidation state of 2.46—mostly match the MUS spectrum. In conclusion, galaxite is present in the MUS.

Metals 2021, 11, 188 14 of 20 Metals 2021, 11, x FOR PEER REVIEW 15 of 22

Figure 8. Linear combination of bixbyite (B) and galaxite (G). The spectrum of the MUS is similar to the linear combination. Metals 2021, 11, x FOR PEER REVIEW 16 of 22 At theedge Figure jump, 8. Linear the combination MUS spectrum of bixbyite ﬁts (B) the and middle galaxite (G). of theThe linearspectrum combination, of the MUS is similar whereas to the linear in the combination. region after the edge, the At the edge jump, the MUS spectrum fits the middle of the linear combination, whereas in the region after the edge, the spectrumspectrum is more is similar more similar to that to that of galaxite.of galaxite.

The linear combinations (Figures 7 and 8) of a 50:50 (mass) mixture of both galaxite and bixbyite, as well as galaxite and hausmannite, are quite similar to the measured spectrum of the MUS V-3. A combination of bixbyite and hausmannite is excluded from further investigation, as the shape is significantly different (see Appendix D). Accordingly, the experimental XANES data was obtained from a 50:50 (mass) mixture of galaxite and bixbyite, as well as galaxite and hausmannite. The obtained spectra are shown in Figures 9 and 10. Figure 10 shows a close-up of the edge-jump.

Figure 9. Experimentally derived X-ray absorption near edge structure analysis (XANES) spectra of a 50 wt.% mixture of galaxite andFigure bixbyite, 9. Experimentally respective derived of galaxite X-ray absorption and hausmannite near edge stru comparedcture analysis to (XANES) the spectrum spectra of obtained a 50 wt.% mixture fromthe of MUS V-3. The galaxite and bixbyite, respective of galaxite and hausmannite compared to the spectrum obtained from the MUS V-3. The pre-edge peak,pre-edge edge peak, jump edge and jump course and course of theof the spectrum spectrum are are very very similar similar for all for three all spectra. three spectra.

Metals 2021, 11, x FOR PEER REVIEW 17 of 22

Metals 2021, 11, 188 15 of 20

Figure 10. Close-up to the edge jump from Figure9. Figure 10. Close-up to the edge jump from Figure 9. 5. Discussion TheThe spectra experimental displayed investigationin Figures 9 and of 10 the show influence that the of pre-edge, Mn on the the solidification, edge jump and and the region after the edge of the MUS and both references are similar. The results suggest especially on the formation of the EnAM LiAlO2 in slags of the six-component oxide 2+ 3+ thatsystem the slag (Li, contains Mg, Al, a Si, mixture Ca and of Mn) Mn is crucial and Mn to understand., confirming This the isEPMA also indispensable analysis. The for combinationsthe phase relations,of galaxite as and well bixbyite as the with reactions an average in this Mn complex oxidation system. state Itof will2.69 also– as well help to as ofpredict galaxite the and slag hausmannite, composition with and improvean average thermodynamic oxidation state modeling.of 2.46 – mostly Slags, match unlike the most MUSgeological spectrum. features, In conclusion, are formed galaxite on a is short present timescale in the MUS. and with high cooling rates. Hence, non-equilibrium thermodynamic modeling will have to be consulted to develop a route to 5. Discussioncreate the desired EnAM. TheIn experimental contrast to the investigation other elements of the in influence this system, of Mn Mn on is redox-sensitive,the solidification, occurring and es- in peciallyseveral on oxidation the formation states of ranging the EnAM from LiAlO +2 to +7.2 in Dueslags to of the the moderate six-component to high oxide oxygen system fugacity (Li,in Mg, the Al, slag, Si, the Ca expected and Mn) oxidation is crucial numbers to understand. are +2, +3This and is +4,also and indispensable mixtures thereof. for the The phasepurpose relations, of this as well research as the was reactions to study in this the complex suppression system. of LiAlO It will2 alsoformation help to in predict Mn-rich the Li-ionslag composition battery recycling and improve slags. The thermodyna determinationmic ofmodeling. the Mn-species, Slags, unlike including most the geologi- oxidation cal statefeatures, formed are informed slags, ison key a short to understanding timescale and this with phenomenon. high cooling rates. Hence, non- equilibriumInvestigations thermodynamic with PXRD modeling and will EPMA have on to Mn-rich be consulted MUS revealto develop that besidesa route to LiAl cre- and ate GCAS,the desired the melt EnAM. contains large grains of Al/Mn-rich oxides. The PXRD results show that theseIn contrast oxides canto the be other best described elements asin spinel-likethis system, compounds. Mn is redox-sensitive, The diffractograms occurring exhibit in severalreflections oxidation in the states range ranging of the from main +2 (311) to +7. diffraction Due to the line moderate of the spinel-structures to high oxygen MnAlfugac-2O4 ity (galaxite),in the slag, Li the1−x Mnexpected2O4, Li oxidation2Mn2O4 and number LiAl5sO are8 (Figure +2, +34 ).and Due +4, to and the non-directmixtures thereof. matching Theof purpose these diffraction of this research lines, thewas best to study explanation the suppression is a spinel of solid LiAlO solution2 formation with in the Mn-rich elements Li, Al and Mn. With increasing Mn concentration within the melt experiments MUS V-1 to

Metals 2021, 11, 188 16 of 20

V-3, there is a shift of the diffraction reﬂection towards galaxite, indicating that the galaxite component is increasing. The amount of LiAlO2 seems to be suppressed compared to an Mn-free melt with similar Li-concentration (Figure4c, black line). Due to the high peak to background ratio, a comparable high amount of amorphous phase can be assumed. The BSE(Z) micrograph observations show large idiomorphous Mn-rich grains (exam- ple see: Figure5), suggesting an early and complex crystallization scenario. EPMA point scan analyses (Table3) show a distinct decrease in the aluminum concentration from the center to the rim of the predominant Mn-rich crystals. At the edge, the aluminum concentration drops nearly to zero. Additionally, there is a split into two components, one relatively Mn-enriched and one relatively Mn-depleted. If the composition of all measurements 1 is calculated as fractions of the virtual compounds Mn0.5AlO2 ( 2 galaxite), LiMnO2 and 1 2+ 3+ Mn0.5MnO2 ( 2 hausmannite) a general formula (Li(2x)Mn (1−x))1+x(Al(2−z),Mn z)O4 can be calculated. From this calculation, a Li-content is derived and used to assess a gradient of the Li-bearing compounds. In accordance with the elemental gradients, a constant decrease of the galaxite fraction from the center to the rim is observed. In contrast, the hausmannite fraction is increasing. The Li-Mn compound fraction stays more or less constant except for a steep increase at the last ~10 µm from the rim. Directly at the rim, a split into a “normal” Metals 2021, 11, x FOR PEER REVIEW 19 of 22 and a Mn0.5MnO2-dominated region can be observed. The increase and decrease of the individual species over the point scans are shown in Figure 11.

1 FigureFigure 11.11. FractionsFractions of the virtual compounds Mn Mn0.50.5AlOAlO2 2(½( 2 galaxite,galaxite, Gal), Gal), LiMnO LiMnO2 (Li-Mn),2 (Li-Mn), and and Mn0.5MnO2 (½1 hausmannite, Hsm) in the grain presented in Figure 5. Mn0.5MnO2 ( 2 hausmannite, Hsm) in the grain presented in Figure5.

6. ConclusionsThis observation indicates that from the beginning to the end of the crystallization, Li is incorporated into the spinel structure. The spinel composition itself changes from a galaxite- In this study, Mn-rich grains in mock-up slags (system: Li2O-CaO-SiO2-Al2O3-MgO- dominated to hausmannite-dominated chemistry. Directly at the rim, the oversaturation MnOx) were characterized to understand the suppression of the LiAlO2 formation in ofMn-rich the melt Li-ion with battery Mn is recycling such that slags. the spinelThe PXRD, solid EPMA solution and segregates XANES data (most suggest probably that duringMn-rich cooling grains downcrystallize to room early temperature) on as a spinel to solid form solution. two different A generic (most stoichiometry, probably spinel- i.e., like) oxides. (Li(2x)Mn2+(1−x))1+x(Al(2−z),Mn3+z)O4 of the solid solution was determined assuming a combi- In this respect, it is interesting that at lower temperatures, the hausmannite converts nation of the virtual components Mn0.5AlO2, LiMnO2 and Mn0.5MnO2. From the spatially to the tetragonal crystal system with low solubility of the spinel compound galaxite as resolved data, it was concluded that the solid solution is relatively Al-rich at the beginning reported by Chatterjee et al. [9]. This could indicate an exsolution of the hausmannite of the crystallization and becomes depleted during the process. The formation of spinel solid solution with Mn and Al seems to scavenge Li from the melt before the LiAlO2 crystallization can begin. In conclusion, the experimental evaluation of mock-up slags has provided valuable insights into the Li, Mg, Al, Si, Ca, Mn and O system, and emphasized the benefit to study model melts and slags. In the future, however, an approach allowing for a faster synthesis of variable composition would be desirable. This approach will help to design suitable EnAM. The extraction of the EnAM from the slag and further processing will be part of subsequent studies. In addition, it is not clear how these early crystals form on a molecular level. The solid solution could be a product of a solid phase process. It could also be driven by the ion-pair formation in the melt. In this respect, it is crucial to evaluate the primary crystallization fields in the system Li2O-Al2O3-MnOx in the presence of the other slag compounds Mg, Si and Ca. Despite this, the influence of the viscosity and the oxygen concentration on the early formation of Mn-rich compounds needs to be studied. The impact of viscosity changes, and pair formation in the ionic melt could be accessed by molecular dynamic modeling.

Metals 2021, 11, 188 17 of 20

component due to crystal lattice incompatibility. The hypothesis is backed by the results from the Mn K-edge analysis, which suggests a mixture of galaxite and Mn2+, Mn3+ oxide spinels. The virtual Li compound would mix into the cubic galaxite-like spinel phase. By combining the above results, a scenario of the large crystal genesis is established. The crystallization starts with a high aluminum galaxite-like composition that is subsequently enriched in Mn during the crystal growth. At the end of the crystallization, the solid solution becomes unstable, indicated by exsolution whiskers with a higher mean atomic number, surrounded by the massive crystal.

6. Conclusions

In this study, Mn-rich grains in mock-up slags (system: Li2O-CaO-SiO2-Al2O3-MgO- MnOx) were characterized to understand the suppression of the LiAlO2 formation in Mn-rich Li-ion battery recycling slags. The PXRD, EPMA and XANES data suggest that Mn-rich grains crystallize early on as a spinel solid solution. A generic stoichiometry, 2+ 3+ i.e., (Li(2x)Mn (1−x))1+x(Al(2−z),Mn z)O4 of the solid solution was determined assuming a combination of the virtual components Mn0.5AlO2, LiMnO2 and Mn0.5MnO2. From the spatially resolved data, it was concluded that the solid solution is relatively Al-rich at the beginning of the crystallization and becomes depleted during the process. The formation of spinel solid solution with Mn and Al seems to scavenge Li from the melt before the LiAlO2 crystallization can begin. In conclusion, the experimental evaluation of mock-up slags has provided valuable insights into the Li, Mg, Al, Si, Ca, Mn and O system, and emphasized the benefit to study model melts and slags. In the future, however, an approach allowing for a faster synthesis of variable composition would be desirable. This approach will help to design suitable EnAM. The extraction of the EnAM from the slag and further processing will be part of subsequent studies. In addition, it is not clear how these early crystals form on a molecular level. The solid solution could be a product of a solid phase process. It could also be driven by the ion-pair formation in the melt. In this respect, it is crucial to evaluate the primary crystallization fields in the system Li2O-Al2O3-MnOx in the presence of the other slag compounds Mg, Si and Ca. Despite this, the influence of the viscosity and the oxygen concentration on the early formation of Mn-rich compounds needs to be studied. The impact of viscosity changes, and pair formation in the ionic melt could be accessed by molecular dynamic modeling.

Author Contributions: A.W. conceived the paper. A.W., U.E.A.F. and T.S. conducted the literature review. All melting experiments were designed and performed by H.Q. and D.G. The chemical bulk analysis was executed by the analysis laboratory of the Institute of Mineral and Waste Processing. The phase analysis (PXRD) and the mineralogical investigation (EPMA) were conducted by T.S. The speciation analysis with XANES and interpretation of the spectra was conducted by A.W. and U.E.A.F. Interpretation, discussion and conceptualization were conducted by all authors. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Clausthal University of Technology in the course of a joint research project, “Engineering and Processing of Artificial Minerals for an Advanced Circular Economy Approach for Finely Dispersed Critical Elements” (EnAM). Acknowledgments: We acknowledge support by Open Access Publishing Fund of Clausthal Univer- sity of Technology. We thank Jörg Wittrock from the Institute of Inorganic and Analytical Chemistry for the idea and implementation of the galaxite synthesis. We thank Joanna Kolny-Olesiak for discussions and proofreading. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study, in the collection, analyses, or interpretation of data, in the writing of the manuscript, or in the decision to publish the results. Metals 2021, 11, x FOR PEER REVIEW 20 of 22

speciation analysis with XANES and interpretation of the spectra was conducted by A.W. and U.E.A.F. Interpretation, discussion and conceptualization were conducted by all authors. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Clausthal University of Technology in the course of a joint research project, “Engineering and Processing of Artificial Minerals for an Advanced Circular Economy Approach for Finely Dispersed Critical Elements” (EnAM). Acknowledgments: We acknowledge support by Open Access Publishing Fund of Clausthal Uni- versity of Technology. We thank Jörg Wittrock from the Institute of Inorganic and Analytical Chem- istry for the idea and implementation of the galaxite synthesis. We thank apl. Joanna Kolny-Olesiak Metals 2021, 11, 188 for discussions and proofreading. 18 of 20 Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study, in the collection, analyses, or interpretation of data, in the writing of the manu- Appendixscript, or in the A decision to publish the results. The conversion from θ-angle to energy space is done by following Bragg’s law, with theAppendix order of A diffraction (n), Planck constant (h), speed of light (c), interplanar distance (d). The conversion from θ-angle to energy space is done by following Bragg’s law, with n·h·c the order of diffraction (n), Planck constantE = (h), speed of light (c), interplanar distance (d). (A1) 𝑛∙ℎ∙𝑐2·d· sin(θ) 𝐸= (1) 2∙𝑑∙sin (𝜃) a = √ 0 For a cubic system, d is deﬁned as: d , with lattice spacing (a0) and For a cubic system, d is defined as: 𝑑= h2, with+ k2 lattice+ l2 spacing (a0) and Miller ² ∙∙ n·h·c Millerindices indices (h, k, l). (hTherefore,, k, l). Therefore, the term theterm is dependentis on dependent the chosen on crystal the chosen and the crystal order and the 2d orderof diffraction. of diffraction.

AppendixAppendix B B

Metals 2021, 11, x FOR PEER REVIEW 21 of 22 Figure A1. Spectra of Mn samples of different oxidation states. The spectra are compared to the measurement of the MUS. FigureOxidation A1. Spectra states of ranging Mn samples for a of better different overview oxidation in contraststates. The to sp Figureectra are6 from compared +2 to to +3. the measurement of the MUS. Oxidation states ranging for a better overview in contrast to Figure 6 from +2 to +3. AppendixAppendix CC

FigureFigure A2. A2.Close-up Close-up of of Figure Figure 44 showing showing the the lamellae lamellae region. region.

Appendix D

Figure A3. Linear combination of hausmannite (H) and bixbyite (B).

Metals 2021, 11, x FOR PEER REVIEW 21 of 22

Appendix C

Metals 2021, 11, 188 19 of 20 Figure A2. Close-up of Figure 4 showing the lamellae region.

AppendixAppendix D D

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